Film blowing lines generally comprise at least one extruder which is connected to a film die head and which serves to melt and feed the plastic granules to the film die head. The polymer melt is distributed uniformly in the film die head and exits through the annular die. The film bubble is then taken off in longitudinal direction and as a result of the increase in pressure inside the film bubble brought about by the supplied air, is inflated to the desired diameter. A calibration basket located between the film die head and the take-off unit supports and guides the inflated film bubble. Once the film bubble has been cooled by the cooling air supplied from the inside and/or outside, the film bubble is laid flat in a take-off unit and is then wound to a reel.
An internal cooling tower serves to cool the film bubble on the inside. The cooling air is supplied to the internal cooling tower via a cooling air duct in the film die head. Dependent on the application, the internal cooling tower is made up of one or more cooling air nozzle levels. These vary in diameter and number. The cooling air is routed through the nozzles from the inside in a controlled manner to the film bubble. The film characteristics are dependent on the quality and quantity of the air.
In DE 25 55 848 A1 and DE 100 29 175 B4, film blowing lines are described that feature an internal cooling device with which the film bubble is cooled from the inside.
Different thermoplastic materials and additives are used to produce film in film blowing lines. During the film blowing process, low-molecular components of the raw materials such as monomers, paraffin or additives diffuse from the film as it is exiting the film die head. They accumulate in the air inside and outside the film bubble. These particles impact against the hot cooling tower and melt, then collect and form droplets. These droplets run down the outside wall of the cooling tower and in the cooling air nozzles right down to the nozzle gap. They then become entrained in the flow of cooling air and are catapulted against the film bubble. Contamination of the film or damage in the form of holes in the film bubble is the result.